Fuel cells provide an environmentally friendly source of electrical current. One form of fuel cell used for generating electrical power includes an anode for receiving hydrogen gas, a cathode for receiving oxygen gas, and an alkaline electrolyte. Another form of fuel cell includes an anode channel for receiving a flow of hydrogen gas, a cathode channel for receiving a flow of oxygen gas, and a polymer electrolyte membrane (PEM) which separates the anode channel from the cathode channel. In both instances, oxygen gas which enters the cathode reacts with hydrogen ions which cross the electrolyte to generate a flow of electrons. Environmentally safe water vapor is also produced as a byproduct. However, several factors have limited the widespread use of fuel cells as power generation systems.
In order to extract a continuous source of electrical power from a fuel cell, it is necessary to provide the fuel cell with a continuous source of oxygen and hydrogen gas. However, with atmospheric air as the direct source of oxygen to the cathode channel, performance of PEM fuel cells is severely impaired by the low partial pressure of oxygen and the concentration polarization of nitrogen, while alkaline fuel cells require a pretreatment purification system to remove carbon dioxide from the feed air. Further, as the average oxygen concentration in a cathode channel with atmospheric air feed is typically only about 15%, the size of the fuel cell must be undesirably large in order to provide sufficient power for industrial applications.
In order to achieve a partial pressure of oxygen through the cathode channel sufficient for the attainment of competitive current densities from a PEM fuel cell system, particularly for vehicular propulsion, it is necessary to compress the air feed to at least 3 atmospheres before the air feed is introduced to the cathode channel. As will be appreciated, the power input necessary to sufficiently compress the air feed reduces the overall efficiency of the fuel cell system. It has been proposed to use polymeric membranes to enrich the oxygen, but such membranes actually reduce the oxygen partial pressure and the reduction in total pressure more than offsets the limited enrichment attainable.
External production, purification, dispensing and storage of hydrogen (either as compressed gas or cryogenic liquid) requires costly infrastructure, while storage of hydrogen fuel on vehicles presents considerable technical and economic barriers. Accordingly, for stationary power generation, it is preferred to generate hydrogen from natural gas by steam reforming or partial oxidation followed by water gas shift. For fuel cell vehicles using a liquid fuel, it is preferred to generate hydrogen from methanol by steam reforming or from gasoline by partial oxidation of autothermal reforming, again followed by water gas shift. However, the resulting hydrogen contains carbon monoxide and carbon dioxide impurities which cannot be tolerated respectively by the PEM fuel cell catalytic electrodes and the alkaline fuel cell electrolyte in more than trace levels.
In prior art PEM fuel cells operating with an autothermal or partial oxidation fuel processor, ambient air is used as the oxidant. This results in a large load of nitrogen having to be heated and then cooled through the fuel processor system. The substantial volume of nitrogen contributes to pressure losses throughout the fuel processor and anode channels, or alternatively to the cost and physical bulk penalties of making those passages larger.
While water recovery from fuel cell exhaust is highly desirable for efficient fuel processor operation, the conventional fuel cell discharges its oxygen-depleted cathode exhaust gas to atmosphere, and thus requires an extra condenser to recover water which then must be vaporized in the fuel processor at a substantial energy cost. This condenser adds to the radiator cooling load which is already a problem for automotive fuel cell power plants in view of the large amount of low grade heat which must be rejected.
The conventional method of removing residual carbon monoxide from the hydrogen feed to PEM fuel cells has been catalytic selective oxidation, which compromises efficiency as both the carbon monoxide and a fraction of the hydrogen are consumed by low temperature oxidation, without any recovery of the heat of combustion. Palladium diffusion membranes can be used for hydrogen purification, but have the disadvantages of delivery of the purified hydrogen at low pressure, and also the use of rare and costly materials.
Pressure swing adsorption systems (PSA) have the attractive features of being able to provide continuous sources of oxygen and hydrogen gas, without significant contaminant levels. PSA systems and vacuum pressure swing adsorption systems (vacuum-PSA) separate gas fractions from a gas mixture by coordinating pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed gas component relative to a less readily adsorbed gas component of the mixture. The total pressure of the gas mixture in the adsorbent bed is elevated while the gas mixture is flowing through the adsorbent bed from a first end to a second end thereof, and is reduced while the gas mixture is flowing through the adsorbent from the second end back to the first end. As the PSA cycle is repeated, the less readily adsorbed component is concentrated adjacent the second end of the adsorbent bed, while the more readily adsorbed component is concentrated adjacent the first end of the adsorbent bed. As a result, a “light” product (a gas fraction depleted in the more readily adsorbed component and enriched in the less readily adsorbed component) is delivered from the second end of the bed, and a “heavy” product (a gas fraction enriched in the more strongly adsorbed component) is exhausted from the first end of the bed.
However, the conventional system for implementing pressure swing adsorption or vacuum pressure swing adsorption uses two or more stationary adsorbent beds in parallel, with directional valving at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks. This system is often difficult and expensive to implement due to the complexity of the valving required.
Further, the conventional PSA system makes inefficient use of applied energy, because feed gas pressurization is provided by a compressor whose delivery pressure is the highest pressure of the cycle. In PSA, energy expended in compressing the feed gas used for pressurization is then dissipated in throttling over valves over the instantaneous pressure difference between the absorber and the high pressure supply. Similarly, in vacuum-PSA, where the lower pressure of the cycle is established by a vacuum pump exhausting gas at that pressure, energy is dissipated in throttling over valves during countercurrent blowdown of adsorbers whose pressure is being reduced. A further energy dissipation in both systems occurs in throttling of light reflux gas used for purge, equalization, concurrent blowdown and product pressurization or backfill steps. These energy sinks reduce the overall efficiency of the fuel cell system.
Additionally, conventional PSA systems can generally only operate at relatively low cycle frequencies, necessitating the use of large adsorbent inventories. The consequent large size and weight of such PSA systems renders them unsuitable for vehicular fuel cell applications. Thus, a conventional PSA unit for oxygen concentration would require an adsorbent bed volume of about 400 L, and an additional installed volume of about 100 L for pressure enclosures and PSA cycle control valves, in order to deliver a product flow containing about 200 L/min oxygen which would be sufficient for a 40 kW fuel cell.
Accordingly, there remains a need for an efficient fuel cell-based electrical generation system which can produce sufficient power for industrial applications and which is suitable for vehicular applications. There also remains a need for compact, lightweight hydrogen and oxygen PSA systems that operate at higher cycle frequencies and are suitable for vehicular fuel cell-based applications.